Technical Field
The disclosure relates to a method for controlling an inertial drive on the basis of pulse trains having variable amplitude and/or frequency
Description of the Related Art
Providing inertial drives, for example piezoelectric stick-slip drives, with electrical signals having a flat and a steep edge in the manner of a sawtooth voltage waveform is known from the prior art.
When the flat signal edge or ramp voltage is applied, the friction member of the drive entrains the slider. When the steep edge is active, slip occurs between the friction member and the slider. On the next flat edge of the pulse train, the friction member is again made capable of entraining the slider.
It has been found that a desired degree of slip between the friction member and the slider is rarely achieved. For example, the slider is entrained a part of the way, the amount of that part being dependent on the force exerted by the actuator, on the mass of the slider, on elasticities in the material and on external forces.
When very small steps are made, the distance moved backwards is greater not only in relation to the step size, but also in absolute terms, with the result that a reduction in the step size leads to an increase in undesired vibrations of the drive.
When the amplitudes of the sawtooth voltage are small, the backwards motion is very much greater on the steep edge than the step itself that is performed. When the amplitude drops below a minimum value, the system being driven does not perform usable steps.
This means that, when controlling a stick-slip drive with a normal sawtooth voltage as known from the prior art, the vibration amplitude is not dominated by the step size itself when the step sizes are small, but rather by the amount of backwards slip on the steep edge of the pulse.
The principle of inertial drives can be seen from FIG. 1. In such drives, an actuator D is provided to which a periodic, sawtooth-like signal is applied, and which produces an acceleration relative to a displaceably mounted runner E frictionally connected to the actuator. When the acceleration of the actuator is low, the runner follows the actuator due to frictional engagement. When the acceleration of the actuator is high, in contrast, the runner slips relative to the actuator as soon as the inertial force of the runner is greater than the frictional force between the runner and the actuator. Macroscopic movements can be realized when a plurality of steps are performed. Inertial drives are a mechanically simple way to position objects over larger distances with high movement resolution.
The direction of motion of the runner can be predefined using the polarity of the sawtooth waveform.
These drives have the disadvantage that vibrations, which are disruptive for high-precision operations, occur again and again due to stepping.
Inertial drives can be controlled with signals having different waveforms. It is important that a high-acceleration phase is followed by a low-acceleration phase in the opposite direction.
A very common waveform is shown in FIG. 2, waveform A. This is a classic sawtooth wave, with alternating flat and steep edges.
The other waveforms B-F in FIG. 2 are control signals typical of the kind used in inertial drives. EP 0 823738 A2 shows two different waveforms.
A common feature of known control signals is that they couple-in disruptive vibrations when moving an object. Even small steps produce large vibrations when known control signals are used.